DNA transcription

DNA Transcription

DNA transcription is a fundamental process in molecular biology where a specific segment of DNA is copied into RNA by the enzyme RNA polymerase. This process is the first step in the expression of genes, leading to the production of proteins that perform various functions in the cell.

Overview

DNA transcription occurs in the cell nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells. The process involves several key steps: initiation, elongation, and termination.

Initiation

The initiation of transcription begins when RNA polymerase binds to a specific region of the DNA called the promoter. The promoter is a sequence of DNA that signals the start of a gene. In eukaryotes, transcription factors are required to help RNA polymerase bind to the promoter. Once bound, the DNA strands unwind, and RNA polymerase begins synthesizing a complementary RNA strand.

Elongation

During elongation, RNA polymerase moves along the DNA template strand, adding ribonucleotides to the growing RNA chain. The RNA strand is synthesized in the 5' to 3' direction, meaning that nucleotides are added to the 3' end of the RNA molecule. The DNA double helix re-forms behind the moving polymerase, and the newly synthesized RNA strand detaches from the DNA template.

Termination

Termination occurs when RNA polymerase reaches a specific sequence on the DNA known as the terminator. This sequence signals the end of transcription. In prokaryotes, termination can occur through rho-dependent or rho-independent mechanisms. In eukaryotes, termination involves the cleavage of the RNA transcript and the addition of a poly-A tail.

Post-Transcriptional Modifications

In eukaryotic cells, the primary RNA transcript, also known as pre-mRNA, undergoes several modifications before becoming mature mRNA. These modifications include:

• 5' capping: The addition of a modified guanine nucleotide to the 5' end of the RNA.
• Splicing: The removal of non-coding sequences called introns, and the joining of coding sequences called exons.
• Polyadenylation: The addition of a poly-A tail to the 3' end of the RNA.

Significance

DNA transcription is crucial for the regulation of gene expression. It allows cells to respond to changes in their environment by producing specific proteins as needed. Errors in transcription can lead to diseases, including cancer and genetic disorders.

See Also

• DNA replication
• Translation (biology)
• Gene expression

References

• Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular Biology of the Cell. 4th edition. New York: Garland Science.
• Lodish, H., Berk, A., Zipursky, S. L., Matsudaira, P., Baltimore, D., & Darnell, J. (2000). Molecular Cell Biology. 4th edition. New York: W. H. Freeman.